The present invention relates to image sensing devices and, more particularly, to image sensing devices that provide for either smaller pixel sizes or increased pixel fill factor.
Active Pixel Sensors (APS) typically include a photo-sensing means with associated active circuitry elements allocated for pixels on an individual basis. These active circuitry elements typically are means to perform a pixel reset function, or some means to transfer charge, means to perform a voltage conversion, or circuitry elements used in amplification. APS devices have been operated in a manner where each line or row of the imager is selected and then read out using a column select signal (analogous to a word and bit line in memory devices respectively). Prior art devices have been disclosed in which all of these components have been located entirely within a single pixel boundary.
Inclusion of these active circuit element components in each pixel reduces the fill factor for the pixel because it takes up area that could otherwise be used for the photodetector. This reduces the sensitivity and saturation signal of the sensor which, in turn, adversely affect the photographic speed and dynamic range of the sensor, performance parameters that are critical to obtaining good image quality. Additionally, inclusion of these active circuit elements within the pixel places a limitation on the minimum size of the pixel, which adversely affects the size and cost of the image sensor.
In order to build high resolution, small pixel APS devices, it is necessary to use sub-tem CMOS processes in order to minimize the area of the pixel allocated to the row select transistor and other parts of the amplifier in the pixel. In essence, it takes a more technologically advanced and more costly process to realize the same resolution and sensitivity APS device when compared to a standard charge coupled device (CCD) sensor. However, APS devices have the advantages of single supply operation, lower power consumption, x-y addressability, image windowing and the ability to effectively integrate signal processing electronics on-chip, when compared to CCD sensors.
One approach to providing an image sensor with the sensitivity of a CCD and the advantages of an APS device, is to improve the fill factor and sensitivity of an APS device by reducing the number of components and interconnect required entirely within a single pixel while maintaining the desired features and functionality of the original pixel design. U.S. patent application Ser. No. 08/808,444 by Guidash discloses a means for fulfilling this need. In this case, the floating diffusion, source follower amplifier, row select transistor, and reset transistor were shared between two row adjacent photodetectors and transfer gates. This provided a pixel with improved fill factor, but eliminated the ability to perform a global electronic shutter without the use of an external mechanical shutter.
In view of the foregoing discussion, it should be readily apparent that there remains a need within the prior art for a method and apparatus that increase pixel fill factor and/or reduces pixel size while simultaneously allowing for global shuttering.
An image sensor comprises (a) a plurality pixels arranged in rows and columns; (b) at least two adjacent pixels in the same row each having a photodetector; (c) a row select mechanism that allows independent selection of the adjacent pixels; and (d) a common output signal node for the two adjacent pixels.